The increase in bandwidth demand for optical links, such as links found in long-haul optical networks, is necessitating a rapid increase in the capacity of optical links. For instance, the per-wavelength capacities of optical links in some optical communication systems are exceeding 100 gigabits per second (Gb/s). Moreover, to meet future capacity demands for optical networks, next generation optical communication systems are being designed to sustain capacities that reach multi-terabits per second (Tb/s). Although the demand to increase bandwidth and throughput continue to grow, designs for the optical systems are often constrained by cost, power, and size requirements.
For example, inaccuracies in carrier-phase estimation and amplitude equalization cause distortions, i.e., the noise enhancements, which reduce the performance of optical communications systems. In the optical communications, different methods are used to reduce the distortion. Those methods are based on a hard decision for determining the phase and amplitude of the received signal. For example, a decision-directed least-mean-square (LMS) method uses the hard decision for determining the error for the updating.
However, different types of distortions can require different types of filters for equalizing the distortion. The optimal combination of those multiple filters is a desirable cost effective option that is usually difficult to achieve. For example, the equalization of chromatic dispersion is usually performed in the frequency domain over a longer response length. In optics, the chromatic dispersion is the phenomenon in which the group velocity of a wave depends on its frequency. The equalization of the chromatic dispersion can be performed using a static filter with constant coefficients.
Conversely, the equalization of the polarization of the optical signal is a rapid time varying process due to the changes in the channel conditions. The equalization of the polarization can be done with dynamic filter that requires a periodic update of its coefficients. The combination of the static and dynamic filters can be challenging.
Accordingly, there is a need to improve the equalization of the optical signal transmitted over the optical channel.